Rheumatoid arthritis (RA) is a chronic inflammatory disease of unknown etiology characterized by synovial inflammation with frequent progression to articular cartilage and bone destruction. IL-10, a cytokine produced by Th2 cells and macrophages, has primarily immunosuppressive and anti-inflammatory properties. The inhibitory effects of IL-10 have been observed in animal models of arthritis where treatment with this cytokine can ameliorate joint inflammation. The purpose of the present study is to determine the safety and tolerability and potential clinical efficacy of subcutaneous doses of recombinant human IL-10 (rHuIL-10) in patients with RA. The study is a phase II, multicenter, double-blind, placebo-controlled, clinical trial of daily (4 mg/d, 8 mg/d, or placebo) or three times week (8 mg/d, 20 mg/d or placebo) subcutaneous rHuIL-10 therapy. The dosing period is twelve weeks for the randomized phase and an additional 12 weeks for the extension study. The study was amended to include an open label portion with a total of 80 weeks of treatment. The main outcomes are the frequency and severity of adverse effects and the American College of Rheumatology Core Disease Measures.